1. Field of the Invention
This invention relates to an interdigital transducer for use in a surface acoustic wave (SAW) filter and in particular to transducers having busbars with edges of non-linear shapes.
2. Description of the Prior Art
Transducers formed from a thin film of metal pattern on a piezoelectric substrate are known. The pattern is shaped so that the transducer has two busbars with a plurality of electrodes extending between the busbars. The electrodes are parallel to one another and each electrode has a break. The transducer is connected within the filter so that when voltage is applied across the busbars, electric fields between the electrodes generate surface acoustic waves. The busbars have a rectangular shape. The surface waves experience reflections at any discontinuities, such as features in the patterns or substrate edges. These reflections produce spurious signals which distort the response of the transducer. Substrate edges are usually cut at special angles to reflect waves into harmless directions. An acoustic absorber is used at critical points to attenuate the unwanted waves. However, these techniques are not directly applicable to unwanted reflections occurring within the transducer itself. These reflections can occur at the boundaries between the free surface region, the region under the busbars and the region containing the electrodes. The surface wave propagation velocity is different in each region. Although reflections from the boundaries between propagation regions are very weak, they do produce significant effects, particularly in the stopband region on the high frequency side of a SAW filter passband.